Creating Your Own Tree
Fault trees can be very time consuming to complete. However, the advantage of performing such an analysis is that it visually links potential faults and allows engineers to very quickly understand interdependencies between systems and faults within the system. Fault trees also have the advantage that they can be completed for a small portion of the system and then integrated to form a larger fault tree for the entire system if necessary.
The first step in creating a fault tree is to identify the root or the “top event" of the logic tree. To avoid confusion, there should be only one top event. If an additional event needs to be analyzed, then a separate fault tree should be developed. All other items in the fault tree should stem down from the single top event.
Every item that could cause the top event should be included in the tree. The items are linked together with a series of logic expressions. It is recommended that the conventional logic gate symbols are used to link the items in formal fault trees.
The most common fault tree blocks are AND and OR gates. These are used to understand the relationships between lower level events. For example, if either of two lower level events could cause the top event, then they will be connected with an OR gate. If, however, both events act together to create the top level event, then the lower level events will be connected with an AND gate.
Events can also be classified. The classifications used will be determined by the system that is being investigated. However, some of the common event classifications that are used are external events, internal events, and conditioning events.